Conventional video signals comprise a time varying component which conveys image structure information referenced to a dc value which establishes the relative brightness of the scene. During transmission of the video signals, the dc reference value may be lost, thus it becomes necessary to reestablish the reference for output to a decoding device, for example. More specifically, as the black and white luma level in the video signal changes, the average picture level varies causing the entire waveform to shift up and down. That is, changes in the average picture level changes the average de offset of the video waveform. More white in the picture moves the blanking level lower while more black moves the blanking level higher. The changing dc offset in the incoming video signal makes it difficult to read the luma and chroma references and, therefore, makes it more complicated to decode the video information.
Generally, there are two categories of video clamping circuits. For one, clamping is activated during predefined time period of every line usually during blanking (backporch) or horizontal sync which disadvantageously requires a timing signal and/or a circuit generating the timing signal is required. Another category of video clamping circuits uses a comparator circuit with a control loop for automatic clamping and does not need the above-mentioned timing signal, however, due to characteristics of the comparator circuit the control loop is not linear. Though this category of video clamping circuits does not require a timing signal, the stability of this circuit is difficult to design and can be sensitive to external parameter change such as output impedance of the video source.